ANS:A - wall or skin friction

Wall or skin friction, also known as shear stress, is the force exerted by a fluid per unit area on a surface with which it is in contact as it flows past that surface. Here's a detailed explanation:

1. Definition:
   • Wall or skin friction arises due to the viscous effects within the fluid as it flows past a solid boundary, such as a wall or a surface.
   • When a fluid flows over a solid surface, the fluid molecules in direct contact with the surface experience a retardation in velocity due to the no-slip condition, where the fluid velocity at the surface is zero.
   • This velocity gradient within the fluid layer adjacent to the surface results in viscous shear stresses, which are transmitted from the fluid to the solid surface.
2. Mechanism:
   • The mechanism of wall or skin friction involves the interaction between adjacent fluid layers as they slide over each other at different velocities.
   • The fluid molecules in immediate contact with the solid surface adhere to it due to molecular forces. As a result, their velocity is zero.
   • The fluid molecules in the next layer above the surface experience a slightly higher velocity due to the shear forces exerted by the molecules in the adjacent layer.
   • This velocity gradient between adjacent fluid layers gives rise to shear stresses, which act tangentially to the surface and are commonly referred to as wall or skin friction.
3. Importance:
   • Wall or skin friction plays a crucial role in various engineering applications, including fluid mechanics, heat transfer, and aerodynamics.
   • In fluid flow analysis, wall friction contributes to the overall pressure drop or head loss in pipelines, channels, and ducts, affecting the flow rate and efficiency of fluid transport systems.
   • In heat transfer, wall friction influences the boundary layer development, convective heat transfer coefficients, and thermal performance of heat exchangers, boilers, and cooling systems.
   • In aerodynamics, wall friction affects the drag force experienced by vehicles, aircraft, and structures moving through a fluid medium, impacting their performance, stability, and fuel efficiency.

In summary, wall or skin friction represents the shear stresses exerted by a fluid on a solid surface as it flows past it. Understanding and quantifying wall friction is essential for predicting and optimizing fluid flow behavior, heat transfer rates, and the performance of engineering systems in various applications.